The invention relates to an additive to an alkaline peroxide-containing bleaching agent for chemical pulp, mechanical pulp, waste paper and/or mixtures thereof, and to a bleaching agent of this type and to a bleaching process.
Bleaching is intended reliably to produce high final brightnesses with the lowest possible investment cost, a minimum of running costs and, as far as possible, no disadvantageous side effects.
In contrast to chemical pulp bleaching, only lignin-preserving bleaching is in principle suitable for brightening mechanical pulp, be it in the form of groundwood, pressure groundwood, refiner mechanical pulp, thermomechanical or chemico-thermomechanical pulp and waste paper. The bleaching agent usually used is hydrogen peroxide (H.sub.2 O.sub.2) In the production of chemical pulp, lignin-removing bleaching with oxygen and/or hydrogen peroxide is also used.
The brownish yellow color of mechanical pulp is caused essentially by lignins, lignin-like phenols and extracts, and degradation products thereof, which form chromophoric systems due to the presence of conjugated double bonds and auxochromic groups. The increase in the brightness without delignification requires specific destruction of the chromophoric systems with a minimum of pulp extraction, since organic substances present in the bleaching medium will increase the chemical oxygen demand (COD).
Brightening processes in lignin-preserving bleaching and their mechanisms are not yet precisely known in detail.
Hydrogen peroxide decomposes by two reaction mechanisms. In the case of homolytic decomposition, which can be represented by the equation EQU H.sub.2 O.sub.2.fwdarw. 2HO.fwdarw.H.sub.2 O+O.sub.2 ( 1),
hydroxide free radicals are firstly formed and react via a chain reaction to form the decomposition products, water and oxygen. This reaction, which is exothermic per se, is normally prevented by the high activation energy for cleavage of the oxygen-oxygen bond in H.sub.2 O.sub.2. However, it can be catalyzed, in particular by heavy metals and compounds thereof, which are frequently present in bleaching liquids. Homolytic decomposition can thus become the major reaction. This is, however, not desirable since this reaction course causes oxidative damage and only has little bleaching effect in the desired sense. In order to prevent this reaction, the presence of peroxide stabilizers and complexing agents in the bleaching process is regarded as being necessary.
The desired reaction of hydrogen peroxide is the dissociation in water in accordance with the equation EQU H.sub.2 O.sub.2 +H.sub.2 O.revreaction.HO.sub.2.sup.- +H.sub.3 O.sup.+( 2),
The equilibrium constant for this reaction at room temperature is 1.78.times.10.sup.-12. The perhydroxide anion (HO.sub.2.sup.-), which is generally regarded as a bleaching reagent, is of importance here. Its concentration can be increased by increasing the H.sub.2 O.sub.2 concentration or by adding alkali and trapping the acid. The latter is the procedure generally carried out, and one speaks of activation of the hydrogen peroxide.
If stabilizers are not used in the case of lignin-removing bleach containing H.sub.2 O.sub.2 in alkali medium, it is not only perhydroxide anions which form from hydrogen peroxide, but also HO free radicals in accordance with equation (1) and further peroxide free radicals, which may, under certain circumstances, result in high-energy singlet oxygen. Traces of heavy metals, in particular, are effective here, which means that it is important that they are eliminated.
The technological requirements for bleaching can thus be summarized as follows: